Hydraulic units, such as fixed and variable displacement axial piston units used in hydrostatic or hydromechanical transmissions emit mechanical vibrations which are induced by rapid compression and decompression of fluid within the displacement units. Frequently, these vibrations are transmitted to the exterior housing components by way of rigid mechanical mountings, causing the housing to vibrate, thus radiating noise to its ambient environment. This radiated hydraulic noise is sometimes subjectively unacceptable and possibly illegal, prohibiting or restricting use of the hydraulics in some applications.
In operation of such units, for example when the units each have nine pistons, there is a changeover from four to five or five to four pistons at a relatively high frequency in each unit which thus results in a 20% force variation in each unit, with the rate of force change controlled by the configuration of a port plate interconnecting the displacement units. Additionally, the transmission is generating torque and a mounting to attenuate hydraulic noise must have the strength to properly support the housing assembly.
This invention pertains to structure to reduce the hydraulically-induced radiated noise by interrupting the vibration transmission path from the hydraulic units to an exterior housing. This is accomplished by assembling the hydraulic units and components associated therewith into a housing assembly and mounting this assembly to an exterior or primary housing by isolation mounting means, with this means being constructed to take up the torque.
The prior art includes Galos U.S. Pat. No. 3,810,519 and Pekar U.S. Pat. No. 3,902,567. The Galos patent shows a transmission drive assembly having a housing assembly mounted to a primary housing with elastomeric material or other suitable material disposed between the housing assembly and the primary housing. The Pekar patent shows a housing assembly for the transmission mounted to another housing by connections including elastomeric material. It is believed that these patents do not show a mounting of a housing assembly to a primary housing by structure which gives the new and improved results embodied in the structure disclosed in this application, including mountings which enable compression of resilient material and flexure of mounting plates to act in response to axially-induced forces and with the same mountings providing large areas of resilient material operable in shear to take up torque generated by the hydraulic units and transverse forces under relatively light loading and with the structure in effect providing a series spring rate by there additionally being compression of a part of the resilient material under heavier loading.